A plane crashes in an area with no landmass and we lose it. We've overfished many species to near-extinction because we completely failed to understand exactly how many fish there actually are. The sea acts as a natural eraser for human habitation, sometimes wiping out all evidence of entire communities within hours. We lose all kinds of cargo into the ocean and haven't the faintest clue where it goes. We're only beginning to figure out how to convert seawater into potable form effectively. The majority of our planet is covered in something we can't even figure out how to build habitats on top of without piling up dirt underneath.
The belief that we have any mastery over this planet seems to humbly disintegrate when looking at our relationship to our oceans